In the half duplex communication such as the PoC service, or the like, each user takes the floor before such user starts the talking. Merely a single user can take this floor. Thus, a plurality of users cannot take the floor simultaneously. As a result, the user who does not take the floor cannot talk while another user who takes the floor is talking, and cannot notify another user of user's own intention.
FIG. 11 shows a schematic view of the PoC service. A user equipment A 1101, a user equipment B 1104, and a user equipment C 1105 communicate with each other through an IMS (IP Multimedia Sub system) Core 1102 and a PoC Server 1103. The IMS Core 1102 handles the call control (SIP; Session Initiation Protocol) and the user entry, and the PoC Server 1103 manages the floor. In FIG. 11, a solid line denotes the voice and Floor control, and a dotted line denotes the SIP (call control).
Since the half duplex talking is carried out in the PoC service, the user can send user's voice to the other end of the line while such user is pressing the talking button after the connection with the other end of the line is established, and the user cannot hear the voice from the other end of the line while such user is pressing the talking button. Therefore, both users must send and get a short message reciprocally by pressing and releasing the talking button to hold a conversation with each other. Also, the PoC service can provide not only the one-to-one talking but also the simultaneous connection with plural persons on the other end of the line in a particular group. In this case, the user's voice is sent from the equipment at which the user is pressing the talking button to remaining equipments.